An optical network is a collection of optical network devices interconnected by links made up of optical fibers. Thus, an optical network is a network in which the physical layer technology is fiber-optic cable. Cable trunks are interconnected with optical cross-connects (OXCs), and signals are added and dropped at optical add/drop multiplexers (OADMs). The optical network devices that allow traffic to enter and/or exit the optical network are referred to as access nodes; in contrast, any optical network devices that do not are referred to as pass-thru nodes (an optical network need not have any pass-thru nodes). Each optical link interconnects two optical network devices and typically includes an optical fiber to carry traffic in both directions. There may be multiple optical links between two optical network devices.
A given fiber can carry multiple communication channels simultaneously through a technique called wavelength division multiplexing (WDM), which is a form of frequency division multiplexing (FDM). When implementing WDM, each of multiple carrier wavelengths (or, equivalently, frequencies or colors) is used to provide a communication channel. Thus, a single fiber looks like multiple virtual fibers, with each virtual fiber carrying a different data stream. Each of these data streams may be a single data stream, or may be a time division multiplex (TDM) data stream. Each of the wavelengths used for these channels is often referred to as a lambda.
A lightpath is a one-way path in an optical network for which the lambda does not change. For a given lightpath, the optical nodes at which its path begins and ends are respectively called the source node and the destination node; the nodes (if any) on the lightpath in-between the source and destination nodes are called intermediate nodes. An optical circuit is a bi-directional, end-to-end (between the access nodes providing the ingress to and egress from the optical network for the traffic carried by that optical circuit) path through the optical network. Each of the two directions of an optical circuit is made up of one or more lightpaths. Specifically, when a given direction of the end-to-end end-to-end_path of an optical circuit will use a single wavelength, then a single end-to-end end-to-end_lightpath is provisioned for that direction (the source and destination nodes of that lightpath are access nodes of the optical network and are the same as the end nodes of the optical circuit). However, in the case where a single wavelength for a given direction will not be used, wavelength conversion is necessary and two or more lightpaths are provisioned for that direction of the end-to-end path of the optical circuit. Thus, a lightpath comprises a lambda and a path (the series of optical nodes (and, of course, the interconnecting links) through which traffic is carried with that lambda).
Put another way, when using GMPLS on an optical network, the optical network can be thought of as circuit switched, where LSPs are the circuits. Each of these LSPs (uni-directional or bi-directional) form an end-to-end path where the generalized label(s) are the wavelength(s) of the lightpath(s) used. When wavelength conversion is not used for a given bi-directional LSP, there will be a single end-to-end lightpath in each direction (and thus, a single wavelength; and thus, a single generalized label).
The term disjoint path is used to describe a relationship between a given path and certain other network resources (e.g., nodes, links, etc.). There are various levels of disjointness (e.g., maximally link disjoint, fully link disjoint, maximally node disjoint, and fully node disjoint; and each can additionally be shared risk group (SRG) disjoint). For instance, a first and second paths are disjoint if the network resources they use meet the required level of disjointness.
Disjoint paths are formed for a variety of reasons, including forming restricted paths and protection paths. Restricted paths are formed to carry traffic that is not to travel through certain network resources for security reasons. Protection paths are used to provide redundancy; that is, they are used as alternate paths to working paths in case of a network failure of some kind. Protection paths are commonly implemented as either: 1) 1+1 protected; 2) 1:1 protected; or 3) 1:N mesh restored. A 1+1 or 1:1 protected path is a disjoint path from node A to node B in the network where one of the paths is a working path, and the other is a protection path. The working path and the protection path are typically established at the same time. In the case of a 1+1 protected path, the same traffic is carried on both paths, and the receiving node selects the best of the paths (i.e., if the one currently selected by the receiving node degrades or fails, that node will switch to the other). In contrast, in the case of a 1:1 protected path, traffic is transmitted on the working path; when a failure occurs on the working path, traffic is switched to the protection path. A mesh restored path from node A to node B is a pair of shared resource group disjoint paths in the network, where one of the routes is a working path and the other is a backup path. The capacity dedicated on the backup path can be shared with backup paths of other mesh-restored lightpaths.
Typically, the paths (e.g., working or protection paths) are established and provisioned manually, for example, by a network administrator. Protection schemes used in a conventional optical network assumes there is only one failure at a time. When a second path failure occurs before the first failed path recovers, particularly, in a 1:N protection scheme, the traffic of the second failed path is dropped because the protection path is being used by the first failed path, until the network administrator can provision another path to take over the traffic of the second failed path.
In addition, an owner of an optical network typically has no discretion on how many working paths can share a 1:N protection scheme. Typically, when a demand for a path with 1:N protection is received, if there is no room for the 1:N protection, the demand is dropped.